include: restore use of signal.h on windows (fixes build)

Change-Id: Ifd983cf50b1ccbe64a45d3b3ad171ef67f608033
Reviewed-on: https://go-review.googlesource.com/3831Reviewed-by: Dave Cheney <dave@cheney.net>

include/u.h

@@ -71,6 +71,10 @@ extern "C" {
 #include <ctype.h>	/* for tolower */
 #include <time.h>
 
+#ifdef _WIN32
+#include <signal.h>
+#endif
+
 /*
  * OS-specific crap
  */